If your looking to gain weight your best bet is to do it cleanly and eliminate any bad foods such as fast food, saturated fats etc. This way you can be assured that you will put on lean weight with minimal fat and will also lead to improving health as well. But the only way to gain weight is to eat more and exceed your calorie requirement, so dont be afraid of putting on fat. Look on the internet for calorie requirement calculators you will find many. This will give you an idea of how much calories you need to eat to maintain your weight, just eat 300-400 more calories than this every day. Also, eat more often, like 5-6 smaller meals a day, this includes eating something before bed : )